Features of Cambrian Niutitang Formation in the Beidoushan ore district,Weng′an County, Guizhou Province

Niutitang Formation has a wide distribution in the Beidoushan ore deposit, Weng′an County, Guizhou Province. It is small in thickness and can be divided into two members. The Lower Member of the Niutitang Formation is described as upper phosphorite member which has phosphorite or phosphate rocks. It has clastic texture in which the particles have some phosphorite scraps, pyrites or glauconites. Its sedimentary structures include lenticular bedding, horizontal bedding, massive bedding and phosphate concretion. The Upper Member of the Niutitang Formation is described as black shale member. It has clay structure. Its sedimentary structures include horizontal bedding and bigger phosphate concretion. The bottom of the black shale member has a polymetallic layer with such elements as nickel, molybdenum and vanadium. The lower part of the black shale member has pyrite lamina or concretion. The middle part has carbonate concretion. The characteristics of Niutitang Formation show a relatively deep water and anoxic sedimentary environment,which represents a transgression layer. The early sedimentary environment of Niutitang Formation was a coastal-shallow continental shelf with strong hydrodynamic force, the late sedimentary environment was a deeper water continental shelf with weak hydrodynamic force, and the polymetallic layer was formed at the biggest flooding surface.